1. Field of the Invention
The present invention relates to a metal sheet used as a wire grid defining a polarizing component for terahertz waves, a wire grid having the metal sheet, and a method for making the wire grid.
2. Description of the Related Art
Wire grids are polarizing components commonly used for optical waves and electromagnetic waves. YOSHIHARA, Kunio “Butsuri Kogaku” (Kyoritsu Shuppan Co., Ltd., 1966 First Edition) p. 216 discloses an example of such wire grids. Japanese Unexamined Patent Application Publication No. 2003-14620 describes a wire grid included in a polarization analyzing apparatus for terahertz waves.
FIG. 1 illustrates a wire grid. As illustrated in FIG. 1, the wire grid for terahertz waves is obtained by arranging long, thin metal strips 1 across a frame 2 at regular intervals. For example, the metal strips 1 may be tungsten wires with a diameter of several tens of μm and are regularly arranged at about a pitch of about 100 μm.
The diameter and pitch of the metal strips are determined according to the wavelength used. For terahertz waves, the diameter of metal strips is in the range of about 10 μm to about 300 μm, and the pitch thereof is in the range of about 30 μm to about 1 mm.
When wire grids are used as polarizers for terahertz waves as described above, the diameter of the wire grids are in the range of about 20 mm to about 100 mm. In a 100-mm-diameter wire grid, in which the length of metal strips is up to about 100 mm, it is difficult to arrange such long and thin metal strips in parallel. That is, since tension tends to cause breakage of the metal strips and variations in distances between adjacent metal strips, it is very difficult to make the wire grid. As a result, the wire grid is very expensive to produce.
On the other hand, there are commercially available wire grids that are made by forming a thin film on a substrate through which electromagnetic waves are transmitted and, for example, etching the thin film to form a fine wiring pattern. In addition, there are commercially available wire grids that are made by dispersing metal particles in a base material, such as resin or glass and, for example, drawing the base material to form fine wires in the base material. In the configuration in which a substrate or a base material is used as described above, since a phenomenon, such as a multiple reflection or interference, occurs depending on the physical property, such as refractive index, reflectance, or absorption index of the substrate or the base material, special steps must be taken to avoid such a phenomenon.